Particle sorting by fluidic vectoring

ABSTRACT

Disclosed are embodiments of apparatus and methods for separating particles in a fluid stream by size. In one illustrative embodiment of an apparatus for separating such particles, a housing is provided, which defines a channel for a fluid stream containing particles. A suction channel is provided, which terminates at a suction port. The suction port is positioned adjacent to the fluid stream. A blowing channel terminating at a blowing port is also provided. The blowing port is positioned adjacent to the suction port such that the suction port is positioned in between the fluid stream and the blowing port. The blowing port and the suction port are configured to create a low pressure region and thereby redirect the first fluid stream towards the low pressure region.

RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(e) of U.S.Provisional Patent Application No. 60/663,813, filed Mar. 21, 2005, andtitled “Particle Sorting by Fluidic Vectoring,” which is incorporatedherein by specific reference.

TECHNICAL FIELD

The present invention relates generally but not exclusively to sortingparticles in a fluid stream.

BRIEF DESCRIPTION OF THE DRAWINGS

Understanding that drawings depict only certain preferred embodiments ofthe invention and are therefore not to be considered limiting of itsscope, the preferred embodiments will be described and explained withadditional specificity and detail through the use of the accompanyingdrawings in which:

FIG. 1 is a cross-sectional view of one embodiment of an apparatus forseparating particles in a fluid stream.

FIG. 2 illustrates a vertical pressure gradient field, generated by anapparatus for separating particles in a fluid stream, with contour linesof constant velocity superimposed thereon.

FIG. 3 is a graph showing the predicted trajectories of water dropletsreleased upstream of the exit of fluid stream at the same location.

FIG. 4 is a graph showing experimentally measured trajectory angles (θ)of water droplets as a function of droplet diameter (in μm).

FIG. 5 is a cross-sectional view of another embodiment of an apparatusfor separating particles in a fluid stream.

FIG. 6 represents one frame of a test video illustrating test results offluidic vectoring using water droplets as test particles.

FIG. 7 is a cross-sectional view of still another embodiment of anapparatus for separating particles in a fluid stream.

FIG. 8 is a cross-sectional view of yet another embodiment of anapparatus for separating particles in a fluid stream.

FIG. 9 is a cross-sectional view of the embodiment shown in FIG. 8,shown after a fluid stream has been redirected around 180 degrees.

FIG. 10 is a cross-sectional view of another alternative embodiment ofan apparatus for separating particles in a fluid stream.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description, numerous specific details are provided fora thorough understanding of specific preferred embodiments. However,those skilled in the art will recognize that embodiments can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In some cases, well-knownstructures, materials, or operations are not shown or described indetail in order to avoid obscuring aspects of the preferred embodiments.Furthermore, the described features, structures, or characteristics maybe combined in any suitable manner in a variety of alternativeembodiments.

Disclosed are embodiments of apparatus and methods for separatingparticles in a fluid stream by size. In one embodiment, as a fluid jetis turned or redirected—i.e., aerodynamically vectored—particles presentin the jet flow experience a resultant force based largely upon theirsize and due to the counteracting effects of pressure and drag on theparticles inertial tendencies. Larger particles will tend to remain onstraighter paths and, thus, can be segregated from smaller particlesthat tend to more closely follow the vectored jet flow. In suchembodiments, a wide range of particle sizes may be separated with asingle device (i.e., single stage) and a small pressure drop. Thisseparation may occur without contact between the particles and surfacesof the separation device. Aerodynamic vectoring may also allow forcollectors of sorted particles to be designed to collect many differentparticle sizes across the continuum of sizes present in the sample. Theparticle sorting methods and devices described herein may be utilized ata wide range of scales and may be used in conjunction with liquids aswell as gases.

In an illustrative method, a first fluid stream or jet is providedhaving particles disposed therein. A port is provided for the firstfluid stream. A second fluid stream is also provided, along with a portfor the second fluid stream adjacent to the first fluid stream port. Alow pressure region is created adjacent to the first fluid stream so asto redirect the first fluid stream towards the low pressure region. Athird fluid stream may also be provided, along with a port for the thirdfluid stream. The third fluid stream port may be positioned adjacent tothe second fluid stream port such that the second fluid stream port ispositioned in between the first and third fluid stream ports. In apreferred implementation, the second fluid stream is a suction streamand the third fluid stream is a blowing stream.

In an illustrative apparatus, a housing is provided defining a channelfor a fluid stream containing particles. A suction channel is provided,which terminates at a suction port. The suction port is positionedadjacent to the fluid stream. A blowing channel terminating at a blowingport is also provided. The blowing port is positioned adjacent to thesuction port such that the suction port is positioned in between thefluid stream and the blowing port. The blowing port and the suction portare configured to create a low pressure region and thereby redirect thefirst fluid stream towards the low pressure region.

An illustrative embodiment of an apparatus for particle sorting viaaerodynamic jet vectoring is depicted in FIG. 1. The embodiment of FIG.1 includes a channel 10 directing a fluid stream 20. Fluid stream 20includes particles 22. Particles 22 may have a variety of sizes, which,as set forth in greater detail below, will determine the ultimatetrajectory of the particles 22 in fluid stream 20. A fluid stream port25 is provided, at which fluid stream 20 exits from channel 10. Ablowing port 30 is also provided, which directs a blowing fluid stream35 therethrough. Likewise, a suction port 40 is provided, which directsa suction fluid stream 45 therethrough. The blowing and suction fluidstreams may be comprised of air, or of any other gas or liquid asdesired.

A low-pressure region 50, indicated generally by the dashed circle inFIG. 1, is generated by suction fluid stream 45 exiting suction port 40adjacent to the exit of fluid stream 20. The low-pressure region 50 isfurther maintained by blowing fluid stream 35 exiting blowing port 30adjacent to suction port 40.

As illustrated in the embodiment of FIG. 1, aerodynamic vectoringinvolves applying secondary flows near the exit plane of a fluid streamor jet, thereby resulting in redirection of the flow upstream of theexit or port. In the embodiment of FIG. 1, steady blowing is providedthrough a first port near the jet port and steady suction is providedthrough a second port near the jet port.

In the embodiment shown in FIG. 1, suction flow is applied immediatelyadjacent to the port of the jet in between the jet and the blowing flow.The blowing and suction ports/flows in this embodiment function togetherto create a low-pressure re-circulation region 50 above the jet 20. Thisarrangement may be configured to prevent ambient fluid from being drawninto the suction port 40 and instead drawing the fluid from the primaryjet 20 through the suction port 40. The low pressure region 50 resultsin the jet 20 turning or being redirected toward the low pressure region50. Aerodynamic vectoring may decrease the pressure drop across the flowchannel 10 due to the presence of the low pressure region 50 near theexit 25. It has been found that the flow rate through the channel tendsto increase when a vectored jet is employed with the blower at aconstant speed.

As described above, it is thought that aerodynamic vectoring occurs dueto a low-pressure region formed along the upper surface of the flowchannel near the exit of the jet. The vertical pressure gradient field,∂P/∂y, for a typical vectored flow is shown in FIG. 2. FIG. 2illustrates a vertical pressure gradient field with contour lines ofconstant velocity superimposed. A particle located in region 62 willencounter a downward pressure force, while a particle in region 72 orregion 74 will experience an upward force. In addition to vectoring theflow, this pressure field can be used to modify the trajectory ofparticles within the flow. A negative pressure gradient (regions 72 and74) indicates that the pressure below a particle is more than above, andthat a particle in such a region location will experience an upwardforce. A positive pressure gradient (region 62) indicates the opposite,such that particles located there will experience a downward force.Other embodiments may be configured such that only region 72 near theexit plane of the jet channel would be present. Possible means toeliminate the pressure gradients downstream of the blowing port arediscussed infra.

When the jet flow contains particulate, each particle in the flow willexperience several forces, including aerodynamic forces (i.e., pressureP), added mass M, drag D, and buoyancy B. The effect of each of theseforces is accounted for in the following particle equation of motion:${m_{p}\frac{\mathbb{d}V_{i}}{\mathbb{d}t}} = {P_{i} + M_{i} + D_{i} + B_{i}}$with$P_{i} = {m_{f}\left\lbrack {\frac{D\quad u_{i}}{D\quad t} - {\nu\quad{\nabla^{2}u_{i}}}} \right\rbrack}_{Y{(t)}}$$M_{i} = {{- \frac{m_{F}}{2}}\frac{\mathbb{d}}{\mathbb{d}t}\left\{ {{V_{i}(t)} - {u_{i}\left\lbrack {{Y(t)},t} \right\rbrack}} \right\}}$D_(i) = −6  π  a  μ(V_(i)(t) − u_(i)[Y(t), t])B_(i) = (m_(p) − m_(F))  g_(i)

The particle in this equation is of radius a and mass m_(p), is locatedat Y(t), and moves with velocity V(t). The term on the left-hand side ofthe first equation present above is the particle inertia, which isbalanced by the four forces on the right. In this two-dimensional flow,i=1, 2 . . . refers to the streamwise x and cross-stream y directions,respectively. The fluid is of kinematic viscosity v and dynamicviscosity μ. The mass of the fluid displaced by the particle is m_(F).The fluid velocity field u must be known to solve for the particle path.

As those having ordinary skill in the art will appreciate, largerparticles experience larger pressure, drag, and buoyancy forces, whileheavier particles have more inertia. By turning or redirecting the fluidflow, the relative magnitudes of these forces will differ for varyingparticle parameters, such as mass, density and/or volume/diameter. Sincethe balance of these forces determines the final trajectory of theparticle, turning the flow leads to a physical separation of particlesof different sizes. Particles of a desired size can then be collecteddownstream in one or more particle collectors or other collectionareas/structures. It should be understood that the term “size”, as usedherein, may refer to a single parameter, or a combination of parameters,that affect the trajectory of a particle within a fluid stream, such asmass, density, or volume/diameter.

A wide variety of blowing and suction combinations may be used inaccordance with the principles of the invention to produce variousalternative configurations. As previously mentioned, a wide variety offluids, whether liquids or gases, may also be used. The equation ofmotion identified above may be solved to predict the trajectory of aparticle of any size and/or density. It should be understood, however,that the drag term may require modification under certain situations(e.g., when the particle is small compared to the mean free path of thefluid).

To illustrate, FIG. 3 shows the predicted trajectory of water dropletsreleased upstream of the exit of the jet at the same location (thecenter of the channel). The diameter of each particle in micrometers isindicated on the figure next to each predicted trajectory. The jet exitor port in the graph is at x=0. The results show that, for a given jetspeed, particles of, for example, a density of 1000 times that of air inthe range 1-40 μm in diameter can be effectively sorted. At higher jetspeeds, this range may increase.

As shown in FIG. 3, particles of a sufficiently small size, or of adensity similar to the fluid in which they move, will tend to follow thejet flow. However, the more massive the particle, the more likely it isto remain on a relatively straight path, even as the jet flow turns.Thus, deliberately redirecting or causing the flow to turn can result ina particle response that is dependent on its size.

FIG. 4 is a graph showing experimentally measured trajectory angles (θ)as a function of droplet diameter (in μm). The graph demonstrates thecorrelation between the size of a particle in a fluid stream and thedegree to which the particle is redirected through the vectoring methodsdescribed herein.

One particular embodiment of an aerodynamic vectoring particle sorter100 is shown in FIG. 5 (not to scale). Particle sorter 100 includes ahousing 105, which may be used to reduce or eliminate externalinfluences on pressures within the device. A particle-laden stream 120enters from the left and travels through channel 110. The suction andblowing flows in this embodiment are both provided by a singlehigh-pressure blower 115. Particles of a variety of sizes may beincluded within the stream 120. To illustrate, FIG. 4 shows threedifferent particles—particles 122, 123, and 124—ranging in size (withtheir relative sizes exaggerated) from small to large, respectively.Particles small enough to follow the vectored flow leave with theexhaust at the top, while larger particles are collected through one ormore output ports and into collectors according to their size on theright. Thus, particle 122 is shown following a trajectory that leads itinto collection port 182, particle 123 is shown following a trajectorythat leads to collection port 183, and particle 124 is shown following atrajectory that leads to collection port 184.

A vent, such as vent 107, may be provided to prevent the jet flow fromattaching to the lower wall of the device. Multiple output ports/bins atdifferent locations may also be used to collect particles of varioussizes. Although three collection ports are shown in FIG. 5, any numberof collection ports greater than, or less than, three may be used. Twoports may be useful in some configurations designed for separating orremoving particles of a minimum (or maximum) size, mass, or otherparameter. More than three ports may be desirable in otherconfigurations, with the maximum number of collection points beinglimited only by installation and logistical considerations. In addition,the collection ports may be placed in a variety of positions. Forexample, one or more such ports may be placed on the top wall of thedevice or, in embodiments wherein the vectoring extends around 180degrees (described in greater detail below), on the back wall behind theexit of the jet.

In some embodiments, the blowing and suction flow rates may be the sameor similar. In such embodiments, it may be convenient to provide asingle high-pressure blower to supply both flows, as also demonstratedby FIG. 5. In the embodiment shown in FIG. 5, the suction path is joinedwith the blowing path, with the blower 115 positioned in between toprovide for both the blowing flow 135 and the suction flow 145.

Although any apparatus available to one of skill in the art may be used,in one embodiment, a variable-speed ring compressor is provided as the“blower” to supply the suction/blowing force. Systems may also bedesigned to correlate the suction and blowing flow rates such that theyare maintained at a particular percentage of the jet flow rate. Thisallows a user to easily vary the jet flow rate while maintaining thepercentage of suction and blowing constant relative to the jet flowrate. Moreover, it should be apparent that embodiments of the inventionprovide for a highly scalable and inherently flexible system in manyother regards, due in part to the number of adjustable inputs, includingjet flow rate, jet vector angle, collector design, etc.

Through experimentation, it has been found that the vectoring angleincreases linearly with the suction flow rate divided by the jet flowrate, independent of the jet velocity. Additionally, the vector anglecan be held constant as the jet flow rate increases by also increasingthe suction and blowing flow rate.

FIG. 6 illustrates test results using water droplets as test particles.A water mister nozzle was added to the aerodynamic jet vectoring jetsetup upstream of the inlet to the channel 110 which directs the jet120. Water droplets were generated with diameters in the range 10-100 μm(the particle size range was determined outside the vectoring facility).The droplets were visualized by illuminating them with a laser sheet andphotographed with a high-speed camera. FIG. 6 represents one frame fromthe resultant video. It is clear that, while the jet is vectoreduniformly, each particle has a unique trajectory due to its size. Whilethese particles do not originate from the same location (resulting inthe particle paths crossing and offset error in FIG. 6), their pathsresemble the predicted trajectories shown in FIG. 3.

In the embodiments presented and discussed thus far, the exit of theblowing slot is parallel to the primary jet. However, other embodimentsof the invention are contemplated in which this is not the case. It hasbeen found that this orientation may limit the vector angle, since thejet flow is pushed downward to some extent by the blowing flow. This isevident in the pressure gradient field shown in FIG. 2. Regions 72, 62,and 74 just downstream and above the jet exit are pressure gradientsgenerated when the blowing flow intersects the jet. Numericalsimulations of aerodynamic vectoring using oscillatory blowing haveshown that angling the slot upward may result in a substantially highervector angle for the same suction and blowing flow rates. According toembodiments design for oscillatory blowing, a single exit port may beprovided adjacent to the jet port. This single port may be configured toprovide both a blowing and a suction flow. In such embodiments, the flowmay oscillate or alternate back and forth from suction to blowing flowsthrough the same port.

Since a larger vector angle results in a wider range of sortingcapability, the actuator may be modified with an angled blowing slot, asshown in FIG. 7. FIG. 7 illustrates an embodiment having a blowing port230 that is angled away at angle θ from the direction at which the fluidstream travels through channel 210 prior to being redirected. Angle θ inthis embodiment is also the angle at which blowing port 230 is angledrelative to the orientation of suction port 240. Although in several ofthe simulations the angle θ was sixty degrees, this angle may varytremendously depending on the parameters of the system and the desiredoutcome. In another simulation, the angle θ was twenty degrees. It wasfound that increasing the angle θ from zero to twenty degrees increasedthe resultant vectoring angle from about 22.5 to about 45 degrees (allelse being equal).

Embodiments disclosed herein may be useful in a variety of fields, suchas powder material processes, sample concentration, cell sorting, airquality monitoring, automotive exhaust distribution measurements, andblood cell sorting, for example. A number of optional features may alsobe added to improve the accuracy or other characteristics of theinvention, such as surrounding the aerosol to be sorted with a “jacket”of clean air. This may result in a stream of particles that originatefrom a more narrow region. As another option, systems may be designedsuch that the jet width H (see FIG. 7) can easily be varied. Still otherembodiments are contemplated in which the distance between the center ofthe channel 210 and the suction port 240, h_(s), and/or the distancebetween the center of the channel 210 and the blowing port 230, h_(b),vary or may be adjustable (see FIG. 7).

FIGS. 8 and 9 illustrate yet another alterative embodiment. Particlesorter 300 is similar to embodiments previously described, except thatit includes an adjustable blowing port 330. In other words, the angle atwhich the blowing port 330 directs fluid relative to the fluid stream320 is adjustable. In the depicted embodiment, the blowing port 330comprises a slot 330 positioned on a rotatable cylinder 331 to therebyallow the angle at which the blowing port 330 directs fluid relative tothe fluid stream 320 to be adjusted by rotating the cylinder 331. Thecylinder 331 has a second slot 332 through which the blowing flow fromthe high pressure source may enter the cylinder 331.

Particle sorter 300 may be used to statically vary the blowing angle inbetween sorting sessions. Alternatively, particle sorter 300 may berotated in the presence of a fluid stream 320 to redirect the fluidstream 320 around a larger angle than would otherwise be possible. It isthought that it may be possible to redirect the fluid stream 320 around180 degrees, as shown in FIG. 9. It is thought that the low pressurearea may be gradually wrapped all the way around the cylinder 331 byrotating the cylinder 331 and will eventually maintain itself in thatmanner to continually redirect the stream 320 around 180 degrees. Thiswould allow for sorting of finer gradations of particle sizes to therebyachieve a finer sorting “signal”.

Still another embodiment is shown in FIG. 10. Particle sorter 400 issimilar to particle sorter 300 shown in FIGS. 8 and 9, except that itincludes a second blowing port 490 directing a second blowing flow 495.It has been found that, under certain circumstances, instability may becreated in the jet 420. In particular, it has been found that a vortexmay sometimes be formed below the jet 420, thereby creating instabilityin the jet 420. To alleviate this problem, second blowing port 490 maybe provided below jet 420. In the depicted embodiment, the secondblowing port 490 is positioned adjacent to jet 420 such that the jet 420is positioned in between the suction port 440 and the second blowingport 490. It is expected that the blowing flow 495 provided by secondblowing port 490 below the jet 420 may help suppress instability in thejet 420.

Each of the channels described herein are examples means for directing afluid stream. Each of the port configurations described herein, whichoperate to create a low pressure region, are examples of means forredirecting a fluid stream to separate particles in the fluid stream bysize. Each of the collection port configurations described herein areexamples of means for sorting particles in the fluid stream by size.

The above description fully discloses the invention including preferredembodiments thereof. Without further elaboration, it is believed thatone skilled in the art can use the preceding description to utilize theinvention to its fullest extent. Therefore the examples and embodimentsdisclosed herein are to be construed as merely illustrative and not alimitation of the scope of the present invention in any way.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. The scope ofthe present invention should, therefore, be determined only by thefollowing claims.

1. A method for separating particles in a first fluid stream, comprisingthe steps of: providing a port for the first fluid stream; providing asecond fluid stream; providing a port for the second fluid streamadjacent to the first fluid stream port; creating a low pressure regionadjacent to the first fluid stream; and redirecting the first fluidstream towards the low pressure region.
 2. The method of claim 1,further comprising: sorting the particles in the first fluid stream bysize.
 3. The method of claim 2, wherein the particles in the first fluidstream are sorted into a plurality of collection bins according to theirrespective sizes.
 4. The method of claim 1, wherein the second fluidstream travels in a direction substantially opposite to the direction oftravel of the first fluid stream.
 5. The method of claim 1, wherein thesecond fluid stream comprises an oscillatory flow that alternates backand forth from suction to blowing flows through the second fluid streamport.
 6. The method of claim 4, further comprising: providing a thirdfluid stream; and providing a port for the third fluid stream, whereinthe third fluid stream port is positioned adjacent to the second fluidstream port such that the second fluid stream port is positioned inbetween the first and third fluid stream ports.
 7. The method of claim6, wherein the second fluid stream is a suction stream and the thirdfluid stream is a blowing stream.
 8. The method of claim 6, wherein thesecond and third fluid streams are both generated from a single blower,wherein a suction force from the blower draws a fluid into the secondfluid stream port to create the second fluid stream, and wherein ablowing force from the blower pushes a fluid out of the third fluidstream port to create the third fluid stream.
 9. The method of claim 6,further comprising: providing a fourth fluid stream; and providing aport for the fourth fluid stream, wherein the fourth fluid stream portis positioned adjacent to the first fluid stream port such that thefirst fluid stream port is positioned in between the second and fourthfluid stream ports.
 10. The method of claim 9, wherein the third andfourth fluid streams both comprise blowing fluid streams.
 11. Anapparatus for separating particles in a fluid stream, comprising: ahousing defining a channel for the fluid stream; a suction channelterminating at a suction port, wherein the suction port is positionedadjacent to the fluid stream; and a blowing channel terminating at ablowing port, wherein the blowing port is positioned adjacent to thesuction port such that the suction port is positioned in between thefluid stream and the blowing port, wherein the blowing port and thesuction port are configured to create a low pressure region and therebyredirect the first fluid stream towards the low pressure region.
 12. Theapparatus of claim 11, further comprising: a blower positioned in fluidcommunication with the blowing channel and configured to provide ablowing stream out of the blowing port.
 13. The apparatus of claim 12,wherein the blower is positioned in fluid communication with the suctionchannel and is configured to provide a suction stream into the suctionport.
 14. The apparatus of claim 11, further comprising: a plurality ofcollection ports positioned downstream of the location at which thefluid stream is redirected to sort particles in the fluid stream bysize.
 15. The apparatus of claim 14, further comprising: a plurality ofcollection bins corresponding to the plurality of collection ports tocollect the sorted particles by size.
 16. The apparatus of claim 11,wherein the blowing port is angled away from the direction at which thefluid stream travels prior to being redirected.
 17. The apparatus ofclaim 11, wherein the blowing port is angled relative to the orientationof the suction port.
 18. The apparatus of claim 11, wherein the angle atwhich the blowing port directs fluid relative to the fluid stream isadjustable.
 19. The apparatus of claim 18, wherein the blowing portcomprises a slot positioned on a rotatable cylinder to thereby allow theangle at which the blowing port directs fluid relative to the fluidstream to be adjusted.
 20. The apparatus of claim 11, furthercomprising: a second blowing channel terminating at a second blowingport, wherein the second blowing port is positioned adjacent to thefluid stream such that the fluid stream is positioned in between thesuction port and the second blowing port.
 21. The apparatus of claim 11,wherein the housing comprises a vent positioned adjacent to the fluidstream.
 22. An apparatus for separating particles in a fluid stream,comprising: a housing defining a channel for the fluid stream; a suctionchannel terminating at a suction port, wherein the suction port ispositioned adjacent to the fluid stream; a blowing channel terminatingat a blowing port, wherein the blowing port is positioned adjacent tothe suction port such that the suction port is positioned in between thefluid stream and the blowing port, wherein the blowing port and thesuction port are configured to create a low pressure region and therebyredirect the first fluid stream towards the low pressure region; ablower positioned in fluid communication with the blowing channel andconfigured to provide a blowing stream out of the blowing port, whereinthe blower is also positioned in fluid communication with the suctionchannel and is further configured to provide a suction stream into thesuction port; and a plurality of collection ports positioned downstreamof the location at which the fluid stream is redirected to sortparticles in the fluid stream by size.
 23. An apparatus for separatingparticles in a fluid stream, comprising: means for directing a fluidstream; means for redirecting the fluid stream to separate particles inthe fluid stream by size, wherein the redirecting means operates bycreating a low pressure region generated from a second fluid stream; andmeans for sorting particles in the fluid stream by size.
 24. Theapparatus of claim 23, wherein the redirecting means operates bycreating a low pressure region generated from the second fluid streamand a third fluid stream.
 25. The apparatus of claim 24, wherein thesecond fluid stream is positioned in between the fluid stream and thethird fluid stream, and wherein the second fluid stream comprises asuction stream and the third fluid stream comprises a blowing stream.